Conventional methods of skim processing involve physical removal of the skim from the melt surface by mechanical means, followed by secondary treatment to reclaim as much of the aluminous metal content as possible.
The handling of the skim in the metal melting or holding furnace has a direct influence on the extent of subsequent recovery of aluminous metal from the skim. When a sufficient layer of skim has accumulated on the melt, it should be removed promptly, not only so that heating may continue efficiently, but more importantly, to minimize or prevent oxidation of the entrained metal.
Skim formation on a melt surface apparently commences as a result of oxide in the original charge floating to the surface with adhering metal. In this original state relatively little oxidation has taken plane, and the layer on the melt may contain as much as 95 percent metal. If heat is supplied through this layer, the layer acts as a barrier to heat transfer and its temperature increases. Oxidation begins to increase and changes in physical form can be noted. Oxidation is promoted by the presence of air or products of combustion in an open hearth furnace. Hot molten aluminum skim will combine preferentially with oxygen to form oxides, but reactions can proceed with nitrogen to form nitrides and with carbon dioxide to form oxides and carbides.
As heating proceeds, the original wet mushy skim can change to a powdery form, in which the aluminum appears to form droplets with powdered non-metallic materials on the surface. This latter physical form has a higher oxidation rate than the wet mushy form. As these reactions are highly exothermic and increase with temperature and mass of the layer, they can quickly get out of control and a reaction known as thermiting takes place. Thermiting results in a rapid loss of metal through oxidation and it is difficult to control.
Skim can also be generated as the result of metal transfer operations. Such skim is generally of the wet mushy type but it behaves similarly to skim originating from melting. Skim can also be generated from fluxing with gases such as chlorine, chlorine-nitrogen or other gases used for metal cleanliness purposes. These operations themselves may ignite the skim giving further oxidation.
In prior-art aluminum melting it has been an object to generate the least amount of non-metallic material and separate as much metal as possible in the unoxidized state from the skim itself without regard for recovering the nonaluminuus constituents of the skim.
In the past, for example, thermiting has been employed intentionally to separate aluminum metal from skim. While thermiting occurs as the result of igniting molten aluminum and using it as a fuel, such burning can be used to separate molten aluminum from a mass of skim, particularly when combined with some form of agitation. Thermiting may be induced by the use of a solid ignition salt flux. Mixtures such as 75 percent sodium chloride-25 percent cryolite, anhydrous aluminum chloride, or proprietary compositions containing an active fluouride are generally employed. Gaseous chlorine may be introduced into the melt below the skim layer in conjunction with such treatment. However, the introduction of such chemicals contaminates the non-aluminous constituents of the skim thereby inhibiting their recovery and use as well as making it difficult to dispose of them because they serve as a source of pollution.
It has been proposed, in one prior-art approach, to handle the skim so as to induce and maintain thermiting or burning under controlled conditions by working the skim in an inclined rotary barrel open to the atmosphere, under oxidizing conditions, thus permitting a certain proportion of the metal content to be consumed, in order to recover the rest. This method has the drawbacks of being technically complicated to operate, and of causing voluminous fumes during tumbling of the thermiting skim.
In a subsequent development, the method was modified by covering the rotary barrel and introducing gaseous chlorine to provide an inert aluminum chloride vapor atmosphere. However, with this method, when the cover is removed, a thermiting residue is exposed to the air, creating fume control and safety problems even more acute than those encountered using an open barrel.
A more recent development has been the rotary salt flux furnace process, which is believed to be presently in use, and which involves placing the aluminous skim or dross inside a rotary barrel furnace, and then adding a predetermined amount of a salt flux in solid form. The furnace is then rotated at a suitable rate of speed to obtain a tumbling or cascading action of the mixture of dross and solid flux to break up large lumps of dross. Heat is then applied to the mixture by means of an oil or gas burner effective to liquefy the flux. After the flux is liquefied, the mixture is subjected to a gentle rolling action at a lower speed of rotation, during which the recoverable molten metal is separated from the dross. The flux is preferably a eutectic mixture of about 55 percent potassium chloride and 45 percent sodium chloride, to which from 2.5 percent to 5 percent of cryolite or other fluoride may be added to promote oxide removal from the metal particles. This method has the disadvantages of applying the burner flame directly to the flux, thus presenting a possible source of air pollution, and also requiring a rather high proportion of flux, amounting to about 50 percent or more by weight of the skim. Also, importantly, the residue is contaminated with flux which can be a major detriment to further recovery, processing or disposal thereof. The installation of the rotary furnace also represents a considerable capital investment.
More recently still, the method of molten flux salt stirring in reclaiming aluminous metal from skim or dross has been provided which helps to avoid many of the disadvantages of flux treatment. Skim removed from a body of molten aluminum is placed in a preheated pot, covered with a molten salt flux and subjected to stirring action. This technique permits the use of much less salt flux, minimizes the emission of fumes from the pot, and improves the recovery of aluminum. However, it has certain disadvantages in common with any flux treatment, as related to contamination of the non-aluminous residue, and usually involves the hot dumping of residue at temperatures high enough to be hazardous and to cause objectionable fuming.
Accordingly, what the art has sought, and by various means has attempted to find, is an effecient method of recovering aluminuous metal from skim material with the least possible cost and difficulty of operation. It is an object of this invention to provide a process for treating molten aluminum hot skim material to not only recover an acceptable amount of aluminum therefrom but to also recover relatively uncontaminated non-aluminous constituents therefrom.
It is a further object of this invention to provide a method for treating non-aluminous constituents of hot skim for subsequent use thereof.
It is also an object of this invention to provide new by-products from a process of treating skim material.